Wormholes in Space

[malloyd]

Originally Posted by Flyndaran

So instant to the traveler, but light speed to the rest of the universe is ok?

This is OK everywhere. Travelling at the speed of light *is* instantaneous to the light, time dilation after all.

[Flyndaran]

Originally Posted by malloyd

This is OK everywhere. Travelling at the speed of light *is* instantaneous to the light, time dilation after all.

I know, but I wasn't sure if there was some other issue I was unaware of. But people aren't light, so I wasn't sure how matter would enter the issue.

[lwcamp]

Originally Posted by Sindri

So we avoid the important variety of time travel by having a time jump as well as a spatial jump, is that right?

That's it.

Originally Posted by Sindri

How does this interact with multiple stops? Would it require something like a central hub that everything goes from or can you link it up more?

A central hub avoids problems with forming time travel paths (or almost forming time travel paths before at least one wormhole in the path blows up). This can be thought of as a tree-like structure, where the central hub (the root node) links up to many other destinations (branches), each of which can link up to many other destinations from sub-branches but, like a tree, you, can't form loops.

Now, there may be ways to get loops in your wormhole transit network. If you form a new-born wormhole, each mouth will initially be at the same space and time coordinates. So this wormhole has no time lag. If you take each mouth to new destinations through the pre-existing wormhole network, you will be connecting the nodes through the new wormhole in such a way that you don't get time travel.

Mostly.

You need to be careful not to move any wormhole in a closed loop network too much to avoid time dilation, or to have any one of the nodes too much deeper in a gravitational well for the same reason (including the galactic gravitational well - things closer to the galactic center experience time more slowly). This can cause time lag to build up, until you have an incipient time machine. You might be able to get around this by monitoring the time lag build-up, and if it gets too dangerous you take down a wormhole link, put one end in a cyclotron for a bit to time dilate it and re-establish a safer set of time changes.

Originally Posted by Sindri

Is travel through a wormhole instantaneous or can there be travel time from the perspective of the traveller?

To a traveler, a wormhole is just space like any other set of space. For a Visser wormhole, which is flat like a door, it would be just like stepping through a door for all local physical measurements. However, if that is too boring for your setting, you could have the wormhole have a very long throat which needs to be traversed slowly (slowly so as to avoid velocity-dependent tidal effects). Now it will take time to travel the distance of the throat.

Luke

[lwcamp]

Originally Posted by Flyndaran

So instant to the traveler, but light speed to the rest of the universe is ok?

There may be methods of travel like this (such as digitizing your mind and sending it to your destination via an encoded beam of light, then re-assembling your mind over there). However, wormholes do not work like that.

A wormhole connects a place in space at a given time to another place in space at another time. Suppose we have a wormhole with one end here on Earth and the other end on another planet 100 light years away and 5 years in the future*. If you step through, you are transported 100 light years away and 5 years ahead in time. When you step back, you are transported the 100 light years back to Earth and 5 light years back in time. In neither case is this light speed to the rest of the universe.

Luke

* In Earth's coordinate frame. You have to be careful to specify the coordinate frame, because to someone moving at a different velocity from Earth relativity will shift the distance and time around, so that from the point of view of these observers you might even be going backwards in their time coordinate. The only constraint is that the square of the distance minus the square of the time jump must be the same for all observers. This quantity is called the interval, and because all observers agree on it it is a member of a set of quantities known as invariants (the rest mass and the time experienced by an object are other useful invariants). If the interval ever goes negative (due to moving the wormhole around, for example), you can make a time machine and the wormhole will explode if chronology protection is in effect.

[Sindri]

Originally Posted by lwcamp

A central hub avoids problems with forming time travel paths (or almost forming time travel paths before at least one wormhole in the path blows up). This can be thought of as a tree-like structure, where the central hub (the root node) links up to many other destinations (branches), each of which can link up to many other destinations from sub-branches but, like a tree, you, can't form loops.

Now, there may be ways to get loops in your wormhole transit network. If you form a new-born wormhole, each mouth will initially be at the same space and time coordinates. So this wormhole has no time lag. If you take each mouth to new destinations through the pre-existing wormhole network, you will be connecting the nodes through the new wormhole in such a way that you don't get time travel.

So are there any ways to keep every location from being connected to every other location? (except for economics)

[lwcamp]

Originally Posted by Sindri

So are there any ways to keep every location from being connected to every other location? (except for economics)

You can say that in your setting the time-balancing is fiddly and requires a lot of attention and maintenance and down-time, so that it is only worth adding a few cross-links in an otherwise tree-like transit network, or none at all.

If your wormholes are so fragile that they are disrupted by the tides of nearby worlds and suns, and if the throat of the wormhole itself has strong tides, the wormhole being transported could be torn to bits as it goes through the established wormhole. This prevents any cross-links using this method (you can always have one node far out on the network-tree make a wormhole and project it across normal space to another friendly node, but keep the end that stays at home circling in a cyclotron to adjust the time dilation of the mouths so that when the projected end arrives you don't have a time machine. Doing this means you will have to wait a very long time, though, on the order of the actual coordinate time of the trip - at least a year per light year of the link).

Perhaps the central hub likes being the central hub since it directs a lot of trade and traffic their way. This makes them rich and powerful. Being powerful, they decide they don't want other people messing with their hub status, and enforce it with lasers and missiles and space marines. Nodes near the hub still get a disproportionate amount of traffic, so even though they are not quite as rich and powerful as the hub they still really like the status quo and are prepared to use their quite considerable power to keep things the way they are. It is only the restive far ends of the network that want to change things, and they do not have the power to resist the authorities. This leads to Adventure!!! and Excitement!!! and other fun stuff for swashbuckling heroes.

Luke

[Sindri]

If wormholes do have long throats is there anything to prevent you from staying in the throat?

In an non-topographically changing universe would the new universes generated by new wormholes have the same fundamental physical constants and otherwise work the same as the original universe?

I've heard that you can create diametric drives with negative mass. What other interesting stuff might you be able to do?

[lwcamp]

Originally Posted by Sindri

If wormholes do have long throats is there anything to prevent you from staying in the throat?

Boredom? Starvation?

In seriousness, not really. It is just space-time like any other. You can have stuff there, including travelers, which is not moving. It might be a good place to put a station. In practice, it might turn out you need something there in order to maintain the funky negative energy stuff.

Some early wormhole designs had a throat that was extremely dangerous - the original Morris-Thorne wormhole had so much gravity that it would blue-shift the cosmic microwave background photons to blow-torch intensity x-rays.

There might be traffic regulations about not stopping in the throat except in case of emergency (or for emergency vehicles) in order to prevent congestion.

Originally Posted by Sindri

In an non-topographically changing universe would the new universes generated by new wormholes have the same fundamental physical constants and otherwise work the same as the original universe?

Our best guess is no, with the possibility we could deliberately make them that way. The initial bit of our universe that we expand is expanded by a process called inflation. The change from the inflationary vacuum to a non-inflationary vacuum is thought to lead to a fairly random set of physical properties for the particles and forces in the new vacuum. However, we could in principle seed the inflating vacuum just on the other side of the wormhole with the vacuum from our own universe, forcing collapse of the inflationary vacuum into the form of vacuum where we can survive, and which leads to stars and galaxies and Earth-like planets.

Originally Posted by Sindri

I've heard that you can create diametric drives with negative mass. What other interesting stuff might you be able to do?

Note that the diametric drive has net zero mass, so is probably prohibited by the quantum limits that prohibit other forms of negative mass without even more positive mass around to balance it. It is also unstable in many ways.

There are Krasnikov tubes, something like FTL railways in space, but if you have wormholes then Krasnikov tubes don't get you anything you don't already have and probably cost a lot more.

There are Alcubierre style warp drives. But the more we look at these, the more we realize how problematic they are (you can't turn them off, they destroy their destination, they destroy the traveler, and in an asymptotically flat space-time like ours you probably can't turn them on, either).

Also, on another note, I just want to make sure that you are aware that wormholes conserve all the usual conserved quantities (energy, momentum, angular momentum, electric charge, and in non-relativistic cases, mass) and do so locally. So a wormhole mouth gains the mass of anything that goes through it (and momentum, electric charge, etc.) and loses the mass (& etc) of everything that comes out of it. Thus, if you find a wormhole out to the middle of intergalactic space where the far end has a mass of only a few hundred hydrogen atoms, you can't put anything through the wormhole with a mass of more than a few hundred hydrogen atoms before the mass at the far end goes negative and the whole structure probably collapses. Preferably, your wormhole ends up inside a massive object so you can grab some mass from outside and stuff the wormhole with it to bulk it up.

Luke

[vicky_molokh]

Originally Posted by lwcamp

and in an asymptotically flat space-time

Just how does one figure if this is so or not-quite-so. How approximate is 'approximately Newtonian gravity' in such context?

Also, what are the walls of a wormhole's throat like? What happens when an object with mass touches / collides with a wall?

[lwcamp]

Originally Posted by vicky_molokh

Just how does one figure if this is so or not-quite-so. How approximate is 'approximately Newtonian gravity' in such context?

This is always going to be an approximation. It is just that over scales of solar systems and galaxies and galactic superclusters, it is a very good approximation. The errors in this approximation are roughly of the order of the general relativistic corrections to Newtonian gravity over a shell (or other closed surface) surrounding your region of interest at distances sufficiently far away that you get as close to Newtonian gravity as possible over that shell. Which, for the case of starts and galaxies and whatnot, makes this a very very good approximation.

Originally Posted by vicky_molokh

Also, what are the walls of a wormhole's throat like? What happens when an object with mass touches / collides with a wall?

There really are no walls. Not in space-time, anyway. The local wormhole traffic authority might put some up. Or maybe the engineers need to put up walls in order to make the wormhole work.

There is one direction where, if you move with a component of your velocity in that direction, you go through the wormhole and end up on the other side. The other two directions are periodic - if you move in the plane defined by those two directions you will eventually circle back around to end up where you started again. If your velocity has components along both the perpendicular and parallel directions to the throat, you will spiral around. This means that if you look in the direction perpendicular to the throat, you will see wildly distorted ring-images of yourself from the light that you emit which comes back around into your eye.

Luke